Effect of CVD ZrB₂ coating thickness on anti-ablation performance of C/SiC composites

In this study, ZrB₂ coatings with different thicknesses were prepared using chemical vapor deposition on SiC (~100 µm in thickness) coated C/SiC (SiC-C/SiC) composites, and the ablation behaviors were investigated using oxyacetylene torch tests for 20 s. SiC-C/SiC composites with ZrB₂ coatings exhibited improved anti-ablation properties compared to uncoated composites, but anti-ablation properties first increased and then decreased with an increase in ZrB₂ coating thickness. Composites with 10 µm ZrB₂ coating exhibited the best anti-ablation properties, and linear and mass ablation rates were reduced by 38% and 61%, respectively, compared to uncoated composites. Anti-ablation properties of composites with thicker ZrB₂ coatings decreased because of cracks and defects in coatings caused by residual thermal stress that resulted from differences in thermal expansion coefficients between the coating and substrate. During oxyacetylene torch tests, ZrB₂ and SiC were oxidized to ZrO₂ and SiO₂ and formed four layers as thermal barriers that provided ablation protection for inner part and prevented it from being further ablated.